1. Field of the Invention
This invention relates to aryl acetylenic monomers and thermoset polymers made from these aryl acetylenic monomers, and to the use of these monomers and polymers as precursors for amorphous carbon.
2. Description of the Related Art
A. Relating to Amorphous Carbon
Amorphous carbon is a high strength, low weight structural material. It is referred to in the literature as glassy carbon, pyrolytic carbon, or simply as carbon. It maintains its mechanical properties at temperatures up to 3000.degree. C., can be formed into shaped structures, and can be included in a wide range of composite materials. These beneficial properties have made amorphous carbon the material of choice for many applications, especially in the aerospace industry where high strength and low weight are critically important.
Amorphous carbon is typically made by pyrolysis of aromatic compounds such as phenolic resins, phthalonitrile resins, etc. Pyrolysis of these compounds into amorphous carbon involves heating the compounds at sufficiently high temperatures (generally above about 450.degree. C.) to break bonds and evolve a variety of gases containing the non-carbon components of these aromatic compounds, such as H.sub.2 O, H.sub.2, NH.sub.3, etc. This results in the formation of a disordered, amorphous network of polycondensed fused aromatic rings with the beneficial properties described above.
Crystalline (i.e. graphitic) inclusions in the amorphous matrix are observed as amorphous carbon is pyrolyzed at temperatures above about 1000.degree. C. This increased ordering of carbon increases the material density, and provides improved structural and electrical properties. Graphitic inclusions improve the stiffness and thermal stability of carbon structures. These improvements in the properties of structures follow increases in the ratio of carbon in crystalline form to carbon in amorphous form in the structure. This ratio in turn follows the length and temperature of the high temperature (&gt;1000.degree. C.) pyrolysis of the structure.
To date, the precursors for and processes for making amorphous carbon have been unsatisfactory to one degree or another. Some precursors are explosive, especially those with ratios of acetylene groups to aryl groups of about 2:1 or higher. Some precursors are made via low yield, multiple step synthetic routes. Moreover, almost all the known precursors result in relatively poor pyrolytic char yield and high porosity, unless carried out under high pressure (&gt;1 atm).
Highly porous materials result from the evolution of large gaseous volumes. In addition to being inherently low yield, highly porous carbon structures have several intrinsic disadvantages, including high gas permeability, poor high temperature stability, poor mechanical properties, and poor chemical inertness. Overcoming these intrinsic disadvantages generally involves some densification of the pyrolytic product.
U.S. Pat. No. 4,284,834 teaches making amorphous carbon with high char yield. However, the complex, multiple step synthesis of the precursor has a very low yield, making this process commercially impractical.
B. Relating to Aromatic Acetylenic Compounds
There are several known compounds with the general formula: ##STR3## where R.sup.1 is --C.tbd.C.phi., and where R.sup.2, R.sup.3, and R.sup.4 are independently selected from the group consisting of --H and --C.tbd.C.phi.. Specifically, the compounds 1,3,5-tris(phenylethynyl)benzene, 1,2,4,5-tetrakis-(phenylethynyl)benzene, and hexa(phenylethynyl)benzene are known. However, to the best of the applicants' knowledge, the prior art has never taught or suggested the use of these compounds as precursors for thermoset polymers or for amorphous carbon.
Furthermore, to the best of the applicants' knowledge, the specific monomers taught and claimed as the monomers of the invention have never been taught or suggested in the prior art.